In accordance with the recent emphasis on improved availability of potable water, and the more stringent drinking water standards instituted by the Environmental Protection Agency (EPA), the need has increased for an economical method of continuously monitoring the organic content of a stream of process water as a measure of its purity.
One conventional method, using the oxidation of carbon to carbon dioxide and measurement of the carbon dioxide, yields a direct measure of the total organic carbon (T.O.C.), but involves unwieldy procedures requiring specially trained operators. As a result, this general approach is costly and not readily adaptable to continuous measurements. Furthermore, this method provides a measurement of all the carbon in the sample, so that additional steps are required to limit the measurement to the dangerous organic carbon.
Another method, utilizing biochemical oxygen demand (BOD) determination, involves seeding and incubation of a discrete sample for a standard 5 day period, and is accordingly entirely unsuitable to real-time processing.
Yet another method utilizing chemical oxygen demand (COD) determination involves elaborate laboratory procedures, entailing reagent supply and waste disposal problems, and is far too elaborate for ready use.
In addition to the defects noted, all these methods have relatively low sensitivity and are subject to operator errors. The present invention exploits the well known fact that organic compounds, particularly those having aromatic or conjugated unsaturated molecular configurations, absorb UV radiation in the region of 2000-3000 A; therefore, by measuring UV absorbtion, these compounds can be measured. For example, it has been shown empirically by H. Tinsley & Co., Ltd., that there is good correlation between UV absorption measurements and the total organic carbon (TOC) in both river water and sewage effluents; effluents having high mineral contents may differ. Studies at the Mellon Institute in Pittsburgh have shown that such measurements can discern trace organic compounds likely to cause taste and odors in drinking water; see Bramer et al, Instrument for Monitoring Trace Organic Compounds in Water, Water & Sewage Works, Aug. 1966, pp. 275-278.
Such measurements are particularly useful in measuring trace organics in the output of sewage treatment systems, where deionization and filtration steps are provided to remove non-organic compounds likely to interfere with the measurements such as minerals, which also absorb UV radiation. In such situations, an order of magnitude better resolution can be achieved than with chemical methods of TOC measurement.
Devices are known which employ UV absorption measurements to yield a measure of the dissolved organics. One such instrument, described in U.S. Pat. No. 3,751,167 issued to George Claus Feb. 23, 1971, passed filtered UV radiation directly through a cuvette containing the sample fluid to a UV responsive photosensor. Such a device is subject to significant drift with age due to fouling and solarization of the cuvette windows, varying source intensity (a particularly difficult problem with inexpensive germicidal lamps) and gradual degradation of the reflectivity of the internal surfaces.
Another device, described in U.S. Pat. No. 3,535,044 issued to H. Seward, Oct. 20, 1970, utilizes a reference sensor in direct view of the source, thus providing a non-attenuated signal which can be used to compensate for variations in source intensity. However, this approach does not solve problems connected with the sample chamber, such as solarization of the sample tube windows, and reflectivity of the instrument's internal surfaces, nor those connected with the stability of the temperature. Nor does Seward show a suitable sensor.
Yet another device, produced by H. Tinsley & Co., Ltd. similar to that described in the Bramer et al paper referred to above, splits the radiation having been transmitted through the sample into visible and UV components, and provides an output proportional to the ratio of the absorbance of the ultraviolet radiation to that of the visible light. Though this approach is effective in rejecting turbidity of the sample as a contributor to UV absorbtion, variations in the UV intensity relative to that of the visible intensity from the source would not be compensated for; a particular problem in mercury lamps, in which the UV/visible ratio varies dramatically with bulb temperature and age. Moreover, solarization of the windows affects only the UV transmission, thus introducing additional errors.
A further problem encountered arises from the unstability and flicker of the arc lamps generally used on such instruments. Both the Seward and the Tinsley devices utilize pulsed measurement techniques to minimize this problem. Despite their inherent instabilities, arc lamps, particularly the low-pressure mercury vapor variety known as germicidal lamps, are an inexpensive and useful source of short-wave UV radiation. With an output consisting primarily of 253.7 nm, such lamps have found wide application in sterilizers and the like.
Devices are known which employ sensors to measure the UV transmittance of the fluid and thereby activate various devices to render the system fail-safe. One such device is described in U.S. Pat. No. 3,566,105 issued to D. Wilhout on Aug. 16, 1968. The Wilhout device uses a UV responsive photodetector mounted on the wall of the fluid container, illuminated by the lamp radiation through the liquid. A similar device is described in U.S. Pat. No. 3,562,520 issued to R. Hippen on Nov. 4, 1968. Devices of this nature are subject to contamination of the sensor surface and solarization of the sensor itself, and are limited in resolution. Therefore, a need exists in the art for an improved ultraviolet transmittance sensor for monitoring the amount of organic material in a fluid sample.
As discussed above, sterilization by UV exposure from a germicidal lamp is a commonly used method for disinfecting fluids by killing all bio-organisms such as viruses and bacteria without the need for added chemicals. In such systems, the efficiency of the sterilization process is a function of the exposure intensity and duration. Therefore, fouling, solarization, and aging of the UV source and its enclosure as well as changes in the absorption characteristics of the fluid can severely degrade the performance of the sterilizer. Moreover, such a reduction in efficiency is often not apparent since the source, if a conventional lamp, may continue to emit visible light. Further, it will be understood that variations in lamp intensity due to aging, line voltage variation and temperature will also affect the reading accuracy. Since a minimum amount of sterilization is required, over the life of the lamp, the prior practice is to supply excess power, sometimes as much as 50% to the lamp when new, thus wasting energy, reducing the usable life of the lamp, and aggravating the aging problem. A need therefore exists in the art for means to accurately measure the UV output of a UV source, and to control it at a constant level.